MDEV-16735 describes how mysql_upgrade fails when alter_algorithm
is set to a value different than 'DEFAULT'/'COPY'. It was marked as
fixed by 0ee08683, but the fix didn't covered the possibility of having
the global value of alter_algorithm set to something different than
'DEFAULT'/'COPY'. To ensure that the upgrade process works properly
regardless the global value of alter_altorithm, this commit force it's
value to 'DEFAULT' (note the quotes) for the mysql_upgrade session.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.